1. Field of the Invention
This invention relates to devices and circuitry for measuring magnetic fields and more particularly to magnetometer circuits for measuring magnetic fields.
2. Description of Related Art
A number of different types of magnetometers are available. However, of the various types presently available, a fluxgate magnetometer is one of the more practical types for the measurement of weak static magnetic fields. Fluxgate magnetometers rely on the saturation of a magnetic core to provide a basis to measure an absolute magnetic field. The current required to bring the core into saturation is the major source of power consumption for the magnetometer. While it is possible to reduce the power consumption of traditional fluxgate magnetometer sensing techniques for low power applications by reducing the sampling or measurement time, these adaptations do pose a few significant design challenges. With the more reliable second-harmonic fluxgate detection scheme, it is not easy to simply reduce the number of excitation waveform cycles; most systems involve some degree of filtering, which requires many excitation cycles to settle. While it is easier to adapt a peak detection scheme to use only a few excitation cycles, as found in U.S. Pat. No. 4,668,100, issued May 26, 1987 to Murakami et al., peak detection fluxgate systems generally have inferior accuracy. In Murakami et al., a toroidal coil is utilized. It is much more straightforward to adapt a frequency-mode magnetometer detection scheme such as that found in U.S. Pat. No. 5,239,264, issued Aug. 24, 1993 to Timothy J. Hawks. In the Hawks patent, a solenoidal coil is used instead of the Permalloy toroidal coil described in the Murakami et al. patent. Using a simple L/R relaxation oscillator with the solenoidal coil, the magnetometer circuit has an almost instantaneous start-up time. As a direct consequence, it is quite simple to gate the oscillator on for short periods of time while retaining the ability to acquire a period of the waveform to measure the magnetic field value. This circuit still suffers from the requirement for a significant peak current to drive the sensor coil.
An earlier additional magnetometer is disclosed in U.S. Pat. No. 4,851,775 issued Jul. 25, 1989 to Kim et al. In the Kim et al. patent a solenoidal sensor coil is utilized. However, the magnetometer of Kim et al. suffers from the same disadvantages as U.S. Pat. No. 5,239,264 for low power applications.
There is a need to reduce the overall power consumption in magnetometers over that which has been available in prior art magnetometers. One of the objects of the present invention is to reduce the amount of power required to measure the magnetic field.